1. Field of the Invention
This invention relates to a piezoelectric actuator and to an enclosure for piezoelectric elements of such an actuator. The actuator of the invention is suitable for use in a fuel injector for an internal combustion engine, particularly a diesel engine.
2. Description of Prior Art
In a known fuel injector as described in the Applicant's granted patent EP 0995901, a piezoelectric actuator is operable to control the delivery of fuel into a combustion space. Such an actuator typically includes a stack of piezoelectric elements or segments, each approximately 80 microns thick at rest. A variable voltage is applied across each element in use; in response to varying voltage across each element, the stack lengthens and contracts along the central axis of the actuator. The changing length of the stack moves an injector valve needle with respect to a valve seat and thereby controls injection of fuel.
More specifically, the piezoelectric elements of the stack are of polarized piezoelectric material interleaved with internal metallic electrodes. The elements of the stack are separated by and bonded to the electrodes, each of which is capable of providing the necessary voltage to two adjacent piezoelectric elements. The electrodes therefore alternate in polarity along the stack. Edges of the electrodes are exposed on the side surfaces of the stack.
The voltage potential between neighbouring electrodes typically varies from +200V to −60V, depending upon the status of the actuator. When the actuator is required to close the injector valve, the piezoelectric stack is lengthened by about 120 microns by the application of +200V to the elements of the stack. The actuator thereby causes the valve needle of the injector to close against the valve seat, blocking the fuel path. To maintain the injector valve in its closed state, that voltage must be applied continually to the stack except when an injection of fuel is required, at which point a negative voltage of −60V is applied to the electrodes. This causes the stack to contract, retracting the injector valve needle from the valve seat to open the fuel path very briefly. A negative voltage is used for speed of retraction.
The actuator is disposed within an accumulator volume in the injector that receives high-pressure fuel in use. The actuator is therefore immersed in fuel throughout its operational life; moreover, when the fuel injection system is in use, the actuator is exposed to fuel at rail pressure cycling from, for example, 200 to 2000 bar. To protect the piezoelectric stack from damage, it is important that the stack is sealed off from the chemical and dielectric effects of fuel within the accumulator volume. These effects are worsened by water contamination that is typically present in the fuel.
Without an effective barrier between the fuel and the piezoelectric stack, surface short-circuiting can occur between the exposed edges of the electrodes that alternate in polarity along the stack. When short-circuiting occurs, the result is a loss of useful function due to alteration of the piezoelectric material in the region of the short-circuit. It is therefore necessary to package the stack within a protective enclosure defining a barrier to prevent intrusion of fuel and/or water due to leakage and/or permeation thereof.
In EP 0995901, for example, the piezoelectric stack is packaged within a coating or sleeve composed of a flexible sealant material. It is known to use an over-moulding technique to encapsulate the stack within a plastics coating, or to apply a sleeve as described in the Applicant's patent application WO 02/061856. Such an enclosure typically comprises a dielectric polymer material that is able to expand and contract with the actuation of the piezoelectric stack; the material must also accommodate strain due to the significant variation of hydrostatic fuel pressure within the accumulator volume that occurs cyclically in successive injection sequences. The enclosure is suitably formed from a fuel-resistant, low-permeability fluoropolymer such as, for example, polyvinylidene fluoride (PVDF) or ethylene tetrafluoroethylene (ETFE).
A polymer barrier will restrict permeation of fuel and water but will not stop it: it has been observed that there may still be ingress of fuel and water into the interface between the piezoelectric stack and the enclosure.
Ingress of fuel and/or water into the actuator may be driven by various mechanisms. One is the potential for chemical diffusion through the enclosure. Another is the strain on the enclosure due to transient fuel pressure cycling within the accumulator volume. Another is the hydrophilic effect of a predominantly unipolar electric field during operation of the injector. In this respect, a fuel injector valve is open only for very short periods of time during engine operation, i.e. when injecting fuel; the valve is closed for the majority of its operational time. In the much longer interim periods between injections, the piezoelectric stack must be continually energised at +200V to ensure that the valve remains closed. Over time, such is the imbalance between closed and open states that the high voltage of the closed state creates a predominantly unipolar electric field in and around the stack. Such a field has been shown to attract water as ions and/or molecules.
The permeation resistance of the barrier material is an important factor in the durability of a piezoelectric stack in a fuel injector. Metallizing a polymer barrier after assembly has been used to reduce the permeability of the barrier, but the present invention seeks to reduce the permeability of the barrier still further.
A dense metal enclosure would be an effective solution to the problem of permeation. However, the use of a metal enclosure is only possible if the enclosure can be made to withstand the strains experienced concurrently during actuation of the piezoelectric stack and during fuel pressure cycling within the accumulator volume of the injector. Also, being conductive, a metal enclosure has to be separated from the surface of the stack; otherwise, it could give rise to short-circuits between neighbouring electrodes in the stack.
The invention arises from the Applicant's efforts to provide an actuator with a metal enclosure around a piezoelectric stack. Whilst the features of the invention are particularly suited to the use of metal for such an enclosure and indeed are intended to enable the use of metal in that application, those features could be used with enclosures of other materials. Consequently, the invention is not limited, in its broadest sense, to a metal enclosure.